Unsaturated compounds, particularly vinyl monomers, can undesirably polymerize at various stages of their manufacture, processing, handling, storage, and use. Vinyl monomers can undergo self-initiated polymerization at elevated temperatures even in the absence of polymerization promoters. Thus, undesired thermal polymerization can be a problem during the purification of vinyl aromatic monomers and during sudden process shutdowns. Undesirable polymerization results in product loss because the valuable monomer end product is consumed in the undesired side reaction. Moreover, polymerization reduces production efficiency as the polymer is deposited on process equipment. This fouling of process equipment may require a shutdown to remove the undesired polymer by physical methods.
The stable free radical, 4-hydroxy-2,2,6,6-tetra-methylpiperidinoxy (HTEMPO), has been used extensively to control free radical polymerization of reactive monomers during the purification, handling, transportation and storage. However, to improve its efficacy as an inhibitor, there are two alternatives. Firstly, the dose of HTEMPO can be increased. However, as the concentration is increased, the dissolved HTEMPO will crystallize especially if the ambient temperature under which the solution is used or stored falls. HTEMPO can also crystallize if the solvency of the hydrocarbon media decreases, for instance, a solution comprising aromatic solvents will have a lower solvency when it comes into contact with aliphatic media. Owing to the low solubility of HTEMPO in aliphatic media, the introduction of an aromatic-based solvent of HTEMPO will result in the precipitation of HTEMPO thereby resulting in the plugging of quills and transfer lines.
Secondly, the conversion of HTEMPO to its hydroxylamine, HTEMPOL, is the other alternative to increasing its polymer inhibiting efficiency. Unlike HTEMPO that is soluble in aromatic hydrocarbon solvents, HTEMPOL is sparingly soluble in hydrocarbon solvents. In applications that involve aqueous media, the water-soluble HTEMPOL can be used with nominal risk of precipitation whereas it will precipitate in hydrocarbon media. Consequently, the use of HTEMPOL as an inhibitor is restricted to stopping premature polymerization in aqueous media.
In prior art, hydrocarbon-soluble hydroxylamines have been used as inhibitors. Due to the presence of hydrogen substituents of the a-carbon atoms relative to the hydroxylamine functional group, said hydroxylamines are therefore unstable. At high operational temperatures associated with the purification and other processes involving vinylic monomers, these unhindered or partially hindered hydroxylamines decompose to yield contaminant byproducts, namely; aldehydes and primary hydroxylamines. As an example, N,N-diethylhydroxylamine will decompose to acetaldehyde and ethylhydroxylamine.
More particularly, this invention addresses inhibition of polymerization in units typically associated with hydrophobic vinylic monomers such as in distillation towers where aqueous-based inhibitors are not very effective or the poor solubilities of the highly polar inhibitors result in the precipitation or recrystallization of said inhibitors when mixed with hydrocarbon media. In equipment in which a hydrocarbon phase is in contact with an aqueous phase, the currently used hydrophilic hydroxylamines preferentially partition into the aqueous phase rather than the hydrocarbon phase. By contrast, the organic-soluble vinylic species that are liable to polymerization partition into the hydrocarbon phase. Owing to this partitioning tendency, the prior art hydroxylamines are not effective polymerization inhibitors.
Thus, a need exists for a hydrocarbon soluble, stable free-radical scavenger.